1. Field of the Invention
This invention relates to communications systems employing time-division techniques and more particularly to a framing and reframing arrangement for use in the maintenance of synchronization between the two ends of such systems.
2. Description of the Prior Art
In prior-art transmission systems, and in particular pulse-code modulation systems such as are quite commonly used in exchange trunk transmission, recovery of the original information requires that the receiving terminal provide a clock signal which has the same frequency and phase as that of the transmitter. In order to accomplish this result, the basic clock frequency is often recovered from the data stream in order to derive a frequency which is the same as that used in generating the original bit stream. In order to obtain the proper synchronization, the transmitted bit stream includes framing information which in most present-day systems is "winking" signal, i.e., a time slot in each frame is reserved for the framing information which appears in the first frame as a pulse and in the second frame as a zero and then repeats. One technique for the recovery of the winking framing information is disclosed in U.S. Pat. No. 2,527,650 to E. Peterson entitled, "Synchronization of Pulse Transmission Systems". An object of the invention was to cause the receiver timing mechanism to drop back in phase by one pulse position for each two successive frames. Another framing technique is disclosed in U.S. Pat. No. 3,742,139, Michael E. Boehly et al., "Framing System for T-Carrier Telephony". In this system, a group of eight time slots is fed to a store and compared with the corresponding eight time slots two frames later. If there is no framing relationship, the circuit steps to the next successive group of eight time slots and repeats the comparison. In neither of the above referenced patents is the problem of "signaling" bits considered which bits contain information other than the regular framing and synchronizing recovery information such as is incorporated in the winking framing signal.
The present invention relates to a multiplexed communication system and more particularly to the receiving timing information which is necessary to properly decode a plurality of time-division multiplexing systems. One prior-art reference in which the recovery of timing information, and in particular reframing, is disclosed is U.S. Pat No. 3,136,861, J. S. Mayo, "PCM Network Synchronization." This subject patent discloses the transmission of "control" pulses in order to indicate the presence or absence of a "stuffed" pulse and, in fact, uses predictive techniques to determine when a control signal should have occurred in the transmitted signal. More particularly, the patent discloses the use of a framing time slot and a variable time slot, the framing time slot being used for the transmission of framing information whereas the variable time slot, which follows the framing time slot, may contain information or it may be a stuffed pulse. With respect to the synchronization recovery and framing circuit it should be noted that that which is disclosed in the subject Peterson patent is that which is used, or its equivalent is used, by Mayo at the receiving terminal. A winking framing pulse is quite commonly used in present-day digital systems. As pointed out by Peterson in U.S. Pat. No. 2,527,650, the use of a framing signal which has a repetition rate of one-half the framing frequency requires that two frames elapse before it can be definitely determined whether the receiver timing is in the correct phase or not. Thus, in the Peterson system, the "slippage" of one time slot only occurs once in two successive frames whenever there is an indication that a framing error has occurred.
In the present invention, a "preview" circuit is employed. This circuit recognizes the state of the bit in the adjacent time slot and uses this to establish the new pattern for the frame comparison circuitry. Thus, the bit stream can be slipped by one time slot each frame and, therefore, the reframing time is reduced.